1. Field of the Invention
The present invention relates to anti-friction linear motion bearing assemblies and, more particularly, to linear motion bearing assemblies configured to affect load bearing characteristics of the assembly.
2. Description of the Related Art
The present invention is directed to improvements in linear motion bearing assemblies. In particular, the improvements relate to linear motion bearing assemblies of the type whereby a bearing carriage or pillowblock houses a linear bearing assembly for linear movement along a support member, such as, for example, an elongated shaft, rail or spline. The inside bore of the carriage or pillowblock houses the linear bearing.
These linear bearing assemblies are used extensively in a wide variety of machines, machine tools, transfer systems and other equipment where machine elements move with respect to one another. These assemblies, typically, include a bearing carriage mounted for movement along a modified Y-beam, I-beam or T-beam shaped rail. See, for example, U.S. Pat. No. 5,431,498 to Lyon, the disclosure of which is hereby incorporated by reference. Other prior art bearing assemblies are contemplated which include a bearing carriage and a ball retainer dimensioned for insertion into the bearing carriage. See, for example, U.S. Pat. No. 5,613,780 to Ng, the disclosure of which is hereby incorporated by reference.
The load bearing capacity and life of an anti-friction bearing system is dependent on factors such as radial clearance and tolerance between the rolling elements and races, operating loads and environment, type of carriage and carriage materials, bearing size and type, bearing lubrication, and machine operating speeds. Typically, a constant low coefficient of friction of a predetermined magnitude is required in those applications where a load can cause the linear motion bearing assembly to skid which impairs rolling performance.
Accordingly, compensating for a load with load deflection, preloading, etc., provides a constant low coefficient of friction, increases rigidity, and, therefore, the guidance accuracy of the bearing assemblies. However, excessive load compensation can cause uneven or overloading of the rolling elements which may result in pitting, spalling or cracking at the bearing surface of the rolling elements, causing premature bearing failure. Therefore, an optimum load compensation is required to enhance bearing service life and performance characteristics, such as accuracy of travel along the shaft and deflection characteristics.
A known method of applying load compensation to a linear bearing assembly is to machine the races of the bearing to very close tolerances to provide a controlled clearance ("precision fit"). This method results in relatively high costs in terms of labor and equipment.
Another conventional method is providing an open type bearing carriage with an adjusting screw which allows the user to manually adjust preload on the bearing. As the screw is manipulated, the inside bore diameter of the carriage deforms to reduce the clearance between the shaft and the bearing creating pressure between the bearing and the shaft. Typically, a plurality of adjusting screws are positioned between the outer races of the bearing and systematically tightened to approach a uniform friction loading over the diameter/length of the bearing. This method is difficult to implement since the screws must be adjusted in an even manner to uniformly alter the resulting pressure on the rolling elements. Additionally, the adjusting screws are subject to loosening due to machine-generated shock and vibration. Permanent strain can occur preventing any further adjustment.
Moreover, in cases where the external load does not overcome the preset preload, unless preset preload is adjusted on the bearing, a greater dynamic friction force is placed on the bearing. The life of the bearing will be compromised. Therefore, in order to optimize smoothness and longevity, the adjusting screws must be adjusted every time load is changed.
Other devices are known for applying load compensation to a linear bearing assembly. U.S. Pat. No. 5,201,584 discloses a linear bearing slide assembly having a longitudinal spring that applies a compressive force against a preloading bar to the slide for providing a preload.
U.S. Pat. No. 5,328,269 discloses an anti-friction bearing preloaded by a hydraulically or pneumatically generated force applied to the exterior of a bearing carriage.
These conventional methods of load compensation are time consuming, labor intensive and require a high degree of precision. The result is inefficiency in achieving economical production of anti-friction bearings having predetermined load bearing characteristics.
Thus, it would be highly desirable to provide a linear bearing assembly that easily and efficiently provides load compensation. Furthermore, it is contemplated that such an assembly controls load deflection characteristics of the assembly through a pressure transducer structure that measures bearing load during operation and maintains optimum load deflection characteristics of the assembly.
Accordingly, it is one object of the present invention to provide a linear motion bearing assembly which is easily and efficiently manufactured to increase the useful life of the linear motion bearing assembly.
It is another object of the present invention to provide a linear motion bearing assembly that controls load deflection characteristics with pressure transducer structure configured to engage deflectable structure of the assembly.